Field of the Invention
The present invention relates to a process and a brazing foil for the production of a metallic honeycomb body which is wound, stacked or twisted from at least partially structured sheet layers.
Metallic honeycomb bodies are used, for example, as carrier bodies for catalytically active material for use in exhaust gas cleaning in connection with motor vehicles. Other uses in comparably fine sheet metal structures such as, for example, in heat exchangers are also possible.
The honeycomb body is subjected to a brazing operation to form a metallic connection of the sheet metal layers. German Published, Non-Prosecuted Patent Application DE 29 24 592 A1 discloses different possible ways of introducing brazing material into a metallic honeycomb body. The way in which the brazing material is introduced into a honeycomb body is dependent on the brazing material involved. If the material is brazing material in powder form, it can be introduced into the honeycomb body through the use of a brazing powder fluidized bed. In order to ensure that the brazing powder remains in the honeycomb body until a brazing procedure has been carried into effect, it is necessary for the honeycomb body to be at least partially coated with an adhesive so that the brazing powder can cling to the adhesive. Such a brazing procedure is also known from International Publication No. WO 94/06594.
It is also known from German Published, Non-Prosecuted Patent Application DE 29 24 592 A1 that the brazing material is introduced into the honeycomb body in the form of a brazing foil, in particular brazing strips. A brazing strip is introduced between the structured sheet metal layers during an operation of winding or stacking the sheet metal layers. When using a brazing strip it is possible to eliminate the use of an adhesive as is required when using a brazing powder. However, the step of introducing a brazing strip between the sheet metal layers results in losses in a prestressing force in the honeycomb body during the brazing procedure so that in part brazed joints are not formed between the sheet metal layers.
In accordance with German Published, Non-Prosecuted Patent Application DE 29 24 592 A1 a further possible way of introducing a brazing material into a honeycomb body provides for the at least partially structured sheet metal layers to be wound, stacked or twisted to form a cylindrical honeycomb body and then for a brazing foil of given thickness to be pressed into at least one end surface of the honeycomb body. In that operation the brazing foil is pressed into the honeycomb body to such an extent that the brazing material is intended to remain in the individual honeycombs during the brazing procedure.
A problem encountered with such a mode of operation is that the brazing foil may not be severed by the edges of the sheet metal layers since otherwise the brazed portions would fall out of the individual honeycombs of the honeycomb body upon thermal expansion of the honeycomb body in the radial direction. Brazing the honeycomb body in the region of the two end surfaces of the honeycomb body also involves problems since, during the operation of heating the honeycomb body with the brazing material to a brazing temperature, air which is to be found in the honeycomb body expands and that results in an increase in pressure within the honeycomb body. That increase in pressure can cause the brazing material to be urged out of the individual honeycombs.